1use crate::fmt;
2use crate::iter::{FusedIterator, TrustedLen, UncheckedIterator};
3use crate::num::NonZero;
4use crate::ops::Try;
5
6/// Creates a new iterator that repeats a single element a given number of times.
7///
8/// The `repeat_n()` function repeats a single value exactly `n` times.
9///
10/// This is very similar to using [`repeat()`] with [`Iterator::take()`],
11/// but `repeat_n()` can return the original value, rather than always cloning.
12///
13/// [`repeat()`]: crate::iter::repeat
14///
15/// # Examples
16///
17/// Basic usage:
18///
19/// ```
20/// use std::iter;
21///
22/// // four of the number four:
23/// let mut four_fours = iter::repeat_n(4, 4);
24///
25/// assert_eq!(Some(4), four_fours.next());
26/// assert_eq!(Some(4), four_fours.next());
27/// assert_eq!(Some(4), four_fours.next());
28/// assert_eq!(Some(4), four_fours.next());
29///
30/// // no more fours
31/// assert_eq!(None, four_fours.next());
32/// ```
33///
34/// For non-`Copy` types,
35///
36/// ```
37/// use std::iter;
38///
39/// let v: Vec<i32> = Vec::with_capacity(123);
40/// let mut it = iter::repeat_n(v, 5);
41///
42/// for i in 0..4 {
43/// // It starts by cloning things
44/// let cloned = it.next().unwrap();
45/// assert_eq!(cloned.len(), 0);
46/// assert_eq!(cloned.capacity(), 0);
47/// }
48///
49/// // ... but the last item is the original one
50/// let last = it.next().unwrap();
51/// assert_eq!(last.len(), 0);
52/// assert_eq!(last.capacity(), 123);
53///
54/// // ... and now we're done
55/// assert_eq!(None, it.next());
56/// ```
57#[inline]
58#[stable(feature = "iter_repeat_n", since = "1.82.0")]
59pub fn repeat_n<T: Clone>(element: T, count: usize) -> RepeatN<T> {
60 RepeatN { inner: RepeatNInner::new(element, count) }
61}
62
63#[derive(Clone, Copy)]
64struct RepeatNInner<T> {
65 count: NonZero<usize>,
66 element: T,
67}
68
69impl<T> RepeatNInner<T> {
70 fn new(element: T, count: usize) -> Option<Self> {
71 let count: NonZero = NonZero::<usize>::new(count)?;
72 Some(Self { element, count })
73 }
74}
75
76/// An iterator that repeats an element an exact number of times.
77///
78/// This `struct` is created by the [`repeat_n()`] function.
79/// See its documentation for more.
80#[stable(feature = "iter_repeat_n", since = "1.82.0")]
81#[derive(Clone)]
82pub struct RepeatN<A> {
83 inner: Option<RepeatNInner<A>>,
84}
85
86impl<A> RepeatN<A> {
87 /// If we haven't already dropped the element, return it in an option.
88 #[inline]
89 fn take_element(&mut self) -> Option<A> {
90 self.inner.take().map(|inner: RepeatNInner| inner.element)
91 }
92}
93
94#[stable(feature = "iter_repeat_n", since = "1.82.0")]
95impl<A: fmt::Debug> fmt::Debug for RepeatN<A> {
96 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
97 let (count: usize, element: Option<&A>) = match self.inner.as_ref() {
98 Some(inner: &RepeatNInner) => (inner.count.get(), Some(&inner.element)),
99 None => (0, None),
100 };
101 f.debug_struct("RepeatN").field("count", &count).field(name:"element", &element).finish()
102 }
103}
104
105#[stable(feature = "iter_repeat_n", since = "1.82.0")]
106impl<A: Clone> Iterator for RepeatN<A> {
107 type Item = A;
108
109 #[inline]
110 fn next(&mut self) -> Option<A> {
111 let inner = self.inner.as_mut()?;
112 let count = inner.count.get();
113
114 if let Some(decremented) = NonZero::<usize>::new(count - 1) {
115 // Order of these is important for optimization
116 let tmp = inner.element.clone();
117 inner.count = decremented;
118 return Some(tmp);
119 }
120
121 return self.take_element();
122 }
123
124 #[inline]
125 fn size_hint(&self) -> (usize, Option<usize>) {
126 let len = self.len();
127 (len, Some(len))
128 }
129
130 #[inline]
131 fn advance_by(&mut self, skip: usize) -> Result<(), NonZero<usize>> {
132 let Some(inner) = self.inner.as_mut() else {
133 return NonZero::<usize>::new(skip).map(Err).unwrap_or(Ok(()));
134 };
135
136 let len = inner.count.get();
137
138 if let Some(new_len) = len.checked_sub(skip).and_then(NonZero::<usize>::new) {
139 inner.count = new_len;
140 return Ok(());
141 }
142
143 self.inner = None;
144 return NonZero::<usize>::new(skip - len).map(Err).unwrap_or(Ok(()));
145 }
146
147 #[inline]
148 fn last(mut self) -> Option<A> {
149 self.take_element()
150 }
151
152 #[inline]
153 fn count(self) -> usize {
154 self.len()
155 }
156}
157
158#[stable(feature = "iter_repeat_n", since = "1.82.0")]
159impl<A: Clone> ExactSizeIterator for RepeatN<A> {
160 fn len(&self) -> usize {
161 self.inner.as_ref().map(|inner| inner.count.get()).unwrap_or(default:0)
162 }
163}
164
165#[stable(feature = "iter_repeat_n", since = "1.82.0")]
166impl<A: Clone> DoubleEndedIterator for RepeatN<A> {
167 #[inline]
168 fn next_back(&mut self) -> Option<A> {
169 self.next()
170 }
171
172 #[inline]
173 fn advance_back_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
174 self.advance_by(n)
175 }
176
177 #[inline]
178 fn nth_back(&mut self, n: usize) -> Option<A> {
179 self.nth(n)
180 }
181
182 #[inline]
183 fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
184 where
185 F: FnMut(B, A) -> R,
186 R: Try<Output = B>,
187 {
188 self.try_fold(init, f)
189 }
190
191 #[inline]
192 fn rfold<B, F>(self, init: B, f: F) -> B
193 where
194 F: FnMut(B, A) -> B,
195 {
196 self.fold(init, f)
197 }
198}
199
200#[stable(feature = "iter_repeat_n", since = "1.82.0")]
201impl<A: Clone> FusedIterator for RepeatN<A> {}
202
203#[unstable(feature = "trusted_len", issue = "37572")]
204unsafe impl<A: Clone> TrustedLen for RepeatN<A> {}
205#[stable(feature = "iter_repeat_n", since = "1.82.0")]
206impl<A: Clone> UncheckedIterator for RepeatN<A> {}
207